With the rapidly declining availability of hydrocarbon fuels, particularly from petroleum sources, there is a great need to extend efforts for the recovery of the petroleum to sources heretofore practically or economically unattractive and to the recovery of hydrocarbon fuels from alternate sources. A major potential source of petroleum, which has heretofore been virtually untapped because of the inability of most refineries to handle such crudes and the inability and expense of recovering them, are heavy oil deposits. Two basic methods have heretofore been applied in the recovery of such heavy oil deposits, namely; in situ combustion and steam injection methods. Both of these techniques have been limited by the fact that both require the burning of substantial amounts of the oil itself, or equivalent fuels, in order to reduce their viscosity and permit production thereof. This is true even with increased prices of oil. For example, to evaluate the economics of steam injection, the oil/steam ratio (OSR) is utilized. The OSR is the ratio of additional oil recovered for each ton of steam injected. Since it is necessary to burn about eight tons of fuel to get one hundred tons of steam, an OSR of 0.08 has a thermal balance of 0; i.e., you burn as much oil to generate the steam as you produce. Generally, wells in the Kern River Field of California operate with an OSR of 0.24, and are abandoned when they get below 0.15.
However, with the decontrol of heavy oil prices several years ago, substantial work has been done and commercial operations are presently under way utilizing steam recovery techniques for the recovery of heavy oil. In addition, the technology has progressed to the point where application of steam technology to other resource areas such as tar sands, diatomaceous earth, oil shale, and even residual light oil are technically feasible. However, until fairly recently, the state of the art techniques for heavy oil production by steam injection have produced only about 40% to 55% of the oil in place. This of course, is close to the ragged edge of being economic and leaves substantial volumes of oil unrecovered.
Most commercial operations, at the present time, are confined to the use of conventional steam boilers for the generation of steam. Usually, the lease crude is used as a fuel. However, when one considers that 80% to 85% of the cost of a steam injection operation is cost of the fuel, this obviously is a major factor. As a result, a number of alternate energy sources, some rather exotic, have been suggested, including petroleum coke, low BTU lignite coal, natural gas, almond hulls and tree prunings, solar energy, etc. However, except for solar energy, all suggested and used sources of energy for steam generation have the same problems and disadvantages.
First of all, conventional steam boilers waste about 19% of the fuel value in stack losses. Considerable work has been done and progress has been made in the elimination of well bore losses by the use of insulated tubing for the injection of steam.
In addition, numerous heavy oil reservoirs will not respond to conventional steam injection since many have little or no natural drive pressure of their own and even when reservoir pressure is initially sufficient for production, the pressure obviously declines as production progresses. Consequently, conventional steaming techniques are of little value in these cases, since the steam produced is at a low pressure, for example, several atmospheres. Consequently, continuous injection of steam or a "steam drive" is generally out of the question. As a result, a cyclic technique, commonly known as "huff and puff" has been adopted in many steam injection operations. In this technique, steam is injected for a predetermined period of time, steam injection is discontinued and the well shut in for a predetermined period of time, referred to as a "soak". Thereafter, the well is pumped to a predetermined depletion point and the cycle repeated. This technique has the disadvantages that it depends for the recovery of oil, solely on a decrease in viscosity of the oil and the steam penetrates only a very small portion of the formation surrounding the well bore, particularly since the steam is at a relatively low pressure.
However, the most formidable problem with conventional steam generation techniques is the production of air pollutants, namely, SO.sub.2, NO.sub.x and particulate emissions. By way of example, it has been estimated that when burning crude oil having a sulfur content of about 2%, without flue gas desulfurization and utilizing 0.3 barrels of oil as fuel per barrel of oil produced, air emissions in a San Joaquin Valley, Calif. operation would amount to about 40 pounds of hydrocarbons, 4,000 pounds of SO.sub.2, 800 pounds of NO.sub.x and 180 pounds of particulates per 1,000 barrels of oil produced. When these figures are multiplied in a large operation and a number of such operations exist in a single field, the problems can readily be appreciated. Consequently, under the Clean Air Act, the Environmental Protection Agency has set maximum emissions for such steaming operations, which are generally applied area wide, and states, such as California where large heavy oil fields exist and steaming operations are conducted on a commercial scale, have even more stringent limitations. Consequently, the number of steaming operations in a given field have been severely limited and in some cases it has been necessary to completely shut down an operation. The alternative is to equip the generators with expensive stack gas scrubbers for the removal of SO.sub.2 and particulates and to adopt sophisticated NO.sub.x control techniques. This, of course, is a sufficiently large cost to make many operations uneconomic. Further, such scrubbers also result in the production of toxic chemicals which must be disposed of in toxic chemical dumps or in disposal wells where there is no chance that they will pollute ground waters.
It has also been proposed to utilize high pressure combustion systems at the surface of the earth. Such a system differs from the low pressure technique to the extent that the water is vaporized by the flue gases from the combustor and both the flue gas and the steam are injected down the well bore. This has been found to essentially eliminate, or at least reduce or delay, the necessity of stack gas clean up and use of NO.sub.x reduction techniques. The mixture conventionally has a composition of about 60% to 70% steam, 25% to 35% nitrogen, about 4% to 5% carbon dioxide, about 1% to 3% oxygen, depending upon the excess of oxygen employed for complete combustion, and traces of SO.sub.2 and NO.sub.x. The SO.sub.2 and NO.sub.x, of course, create acidic materials. However, potential corrosion effects of these materials can be substantially reduced or even eliminated by proper treatment of the water used to produce the steam. There is a recognized bonus to such an operation, where a combination of steam, nitrogen and caron dioxide are utilized, as opposed to steam alone. In addition to heating the reservoir and oil in place by condensation of the steam, the carbon dioxide dissolves in the oil, particularly in areas of the reservoir ahead of the steam where the oil is cold and the nitrogen pressurizes or repressurizes the reservoir. In fact, in certain types of reservoirs it is believed that the nitrogen creates artificial gas caps which aid in production. As a result of field tests, it has been shown that the high pressure technique results in at least a 100% increase in oil production over the use of steam alone and shortening the time of recovery to about two-thirds of that for steam injection alone. Such tests have generally been confined to injection of steam utilizing the "huff and puff" technique, primarily because results are forthcoming in a shorter period of time and comparisons can be readily made. However, utilization of the high pressure technique in steam drive operations should result in even further improvements. A very serious problem, however, with the currently proposed above ground high pressure system is that it involves a large hot gas generator operating at high pressures and high temperatures. This creates serious safety hazards and, when operated by unskilled oil field personnel, can have the potential of a bomb. In order to be effective, for steam injection, the power output of the combustor should be at least about 7 MM Btu/hr. In order to be useful in a sufficiently large number of reservoirs, the output pressure must be above about 300 psi. The combustor must also be precisely controlled so as to maintain flame stability and prevent flame out, turbulent flow, etc. Such control must also be exercised in feeding and maintaining proper flow of fuel and combustion supporting gas and combustion stoichiometry for efficient and complete combustion, thereby eliminating incomplete combustion with the attendant production of soot and other particulate materials, since excessive amounts of combustion supporting gas for stoichiometric combustion could contribute to corrosion and excessive amounts of fuel result in incomplete combustion and the production of soot and other particulates. A further problem is the construction of the combustor and its operation to prevent rapid deterioration of the combustion chamber and the deposition of carbonaceous materials in the walls of the combustion chamber. Thus, proper cooling of the combustion chamber is necessary, as well as protection of the walls of the combustion chamber. Efficient evaporation and control of the water are also necessary to produce dry, clean steam. Unless the combustor is properly controlled, in addition to introducing the water into the flue gas properly, the water will prematurely dilute the combustion mixture, resulting in incomplete combustion and creation of the water-gas reaction, as opposed to combustion, and prematurely cool the combustion mixture, again producing excessive soot and particulates. The production of solids is particularly serious when utilizing fuels which produce solid residues, such as ash producing coal, lignite, etc. Consequently, to prevent plugging of the formation, the use of fuels which produce solid residues has been confined to indirect techniques of steam generation, where steam is produced in a conventional boiler and the flue gas is not used.
It is therefore and object of the present invention, to overcome the above-mentioned and other disadvantages of the prior art. Another object of the present invention is to provide an improved method and apparatus for the generation of steam for hydrocarbon recovery which reduces heat losses. Another and further object of the present invention is to provide an improved method and apparatus for generating steam for hydrocarbon recovery capable of pressurizing and/or repressurizing petroleum reservoirs. A still further object of the present invention is to provide an improved method and apparatus for generating steam for hydrocarbon recovery which greatly reduces or delays environmental pollution. Yet another object of the present invention is to provide an approved method and apparatus for generating steam for hydrocarbon recovery which is safe to use at the surface of the earth. Another object of the present invention is to provide an improved method and apparatus for generating steam for hydrocarbon recovery including a combustor having a high power output. A further object of the present invention is to provide an improved method and apparatus for the production of steam for hydrocarbon recovery capable of operating at a high pressure. Another and further object of the present invention is to provide an improved method and apparatus for the production of steam for hydrocarbon recovery, including a combustor having a high combustion stability and combustion efficiency. A still further object of the present invention is to provide an improved method and apparatus for the generation of steam which utilizes fuels which produce solid residues. Another and further object of the present invention is to provide an improved method and apparatus for generating steam, utilizing a fuel which produces solid residues, in which the solid residues are effectively removed and clean flue gas is advantageously mixed with the steam. Another and further object of the present invention is to provide an improved method and apparatus for the generation of steam for hydrocarbon recovery capable of producing clean, dry steam. A further object of the present invention is to provide an improved method and apparatus for the generation of steam for hydrocarbon recovery capable of efficient and complete production of steam. Yet another object of the present invention is to provide an improved method and apparatus for the generation of steam for hydrocarbon recovery wherein water for the production of steam is introduced in a manner which prevents the interference of the water with combustion and effectively mixes the water with combustion products. These and other objects of the present invention will be apparent from the following description.